When a well is made to recover natural resources existing in a subterranean formation or area, such as gas, petroleum, and water, various fluids for processing subterranean formations are used. Examples of such a subterranean formation processing fluid include mud water used for excavation, a cementing fluid used for the reinforcement of a water well, and a fracturing fluid used for hydraulic fracturing. A thickener is used for a majority of these fluids.
Use of a thickener upon excavation imparts viscosity to mud water. By circulating such mud water, the scraped rock stone cuttings are transported from the bottom hole to the ground. At the time, by the action of a thickener, mud water has a thixotropic nature (a property in which mud water becomes highly viscous gel if it is left at rest, but the viscosity is decreased, as the velocity of a flowing fluid is increased), and thus, even if the operation is suspended, cuttings in the mud water are not immediately precipitated, but the cuttings can be transported to the ground. In addition, mud water forms a side wall and prevents the collapse of subterranean formations or the outflow of the mud water. Hence, mud water, which can form a thin mud wall and has a small amount of water dehydrated, is preferable. Depending on a subterranean formation and the depth thereof, and taking into consideration salt tolerance or heat resistance, the mixing of sewage water is adjusted.
Cementing fluid is mainly used for filling cement between a casing and a side wall to fix the casing, after a well has been made. If cementing is improperly carried out, since it is likely to lead to the abolition of a well, the leakage of gas, etc., a high-quality cementing fluid is used. As a thickener for such a cementing fluid, hydroxyethyl cellulose, polyvinyl alcohol and the like, which are insusceptible to metal ions, have been used.
Fracturing fluid is a liquid used for stimulating a gas or oil storing layer to promote generation of such gas or oil. For example, since gas or oil is present in a shale layer in a state in which it remains or adsorbs on rock stones, it causes poor flowing and thus it is hardly removed. Hence, the mining of shale gas or oil is generally carried out as follows. That is, first, a well is filled with a fracturing fluid, and a pressure is then applied to crush rock stones and make cracks. At that time, spherical sands each having a size of approximately 0.5 mm, called “proppants,” which are dispersed in the fracturing fluid, are pushed into the cracks, and even after termination of such pressurization, the proppants serve as a supporting body, and are used such that the cracks are not closed. Subsequently, the fracturing fluid, the viscosity of which is decreased as a result of the decomposition of a thickener comprised in the fracturing fluid, is recovered to the ground. Thereby, the flow channel for gas or oil is ensured. Guar gum has been often used as a thickener for the fracturing fluid, since it is able to skillfully control an increase in the viscosity by crosslinking and a decrease in the viscosity by decomposition. Moreover, in order to enhance the dispersibility of proppants, polylactic acid fibers may be mixed into the fracturing fluid.
On the other hand, as components of thickeners for such an intended use, fine cellulose fibers (Patent Literature 1), and a composition for processing subterranean formations, which comprises cellulose nanocrystals produced by an acid hydrolysis method, have been proposed (Patent Literature 2, Patent Literature 3).
Furthermore, in recent years, an innovative method for producing nano-sized ultrafine cellulose fibers, while maintaining the crystallinity of cellulose, has been developed (Non Patent Literature 1). By this method, an aldehyde group or a carboxy group is introduced into the surface of cellulose fibers, and thereafter, fine cellulose fibers are generated by mechanical processing. The ultrafine cellulose fibers obtained by this method have high crystallinity, and also, fibers longer than cellulose nanocrystals can be obtained, and can be converted to high-viscosity gel in water. It has been proposed to use the ultrafine cellulose fibers produced by this method in thickeners for cosmetic products, or in thickeners for excavation (Patent Literatures 4 and 5). In general, it is considered that the thickening properties of a dispersion of ultrafine cellulose fibers can be increased by promoting the miniaturization of cellulose fibers to increase surface area, and also by increasing the contact of fibers with one another. As such, in order to increase the viscosity of a fluid used for excavation, it has been proposed to use ultrafine cellulose fibers having a fiber width of 100 nm or less (Patent Literature 4).